1. Field of the Invention
The present invention relates to a lithium polymer battery with a reinforcement layer and its manufacturing method, and more particularly to a lithium polymer battery casing with an electrically insulative and thermally conductive reinforcement layer having high strength, and a method for manufacturing the same.
2. Description of the Related Art
As is generally known in the art, lithium polymer batteries have an electrode assembly including a separator between a positive electrode plate and a negative electrode plate. The separator not only isolates the positive and negative electrode plates, but also functions as an ion conduction medium, that is, an electrolyte. Such a separator may be formed from a gel type polyelectrolyte and is manufactured in a state where a high polymer is infused with an electrolyte in order to improve ion conductivity. Besides the improved ion conductivity, the gel type polyelectrolyte has a strong bonding property with electrodes, excellent mechanical properties, is easy manufacture and so forth. A typical gel type polyelectrolyte is a polyvinylidene fluoride (PVDF) based electrolyte commercially available from Bellcore Corporation which is produced by mixing a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), a plasticizer and inorganic additives, forming a film and then infusing the film with an electrolyte to gelatinize the film.
A comparison between characteristics of a lithium polymer battery and a lithium ion battery is as follows:
First, since the lithium polymer battery may be constructed in a lamellar structure, it does not necessarily employ a winding process which is required for the manufacture of a lithium ion battery. Thus, an electrode assembly may take a form in which a plurality of plates are laminated, and may be made in a suitable form for an angular structure. Of course, the lithium polymer battery may employ an electrode assembly in a wound form.
Second, an electrolyte of the lithium polymer battery is completely injected within an integrated electrode assembly so that the electrolyte is minimally exposed to the exterior of the battery.
Third, since the lithium polymer battery itself may have a lamellar structure, pressure does not have to be applied when it is formed in an angular shape. Therefore, a battery casing may be made using a thin flexible pouch instead of a thick hard angular or cylindrical can.
If such a flexible pouch is used as the casing of the lithium polymer battery, a thickness of the casing may be significantly smaller than a battery using a can. This allows more electrode assemblies to be received within the same volume, providing for increased battery capacity. Also, since the casing is flexible, the battery may be easily manufactured in a desired shape, allowing it to be easily mountable to a variety of external sets.
In spite of battery capacity increase and workability in various forms, however, the pouch type lithium polymer battery may be weak which may cause many accompanying problems. In the conventional lithium polymer battery, for example, the pouch type casing is likely to be punctured when it is stabbed by a sharp object, such as a needle or a nail, and is easily torn when it is bitten by a pet or other animals. Moreover, if a sharp object pierces the casing and comes in contact with the internal electrode assembly, a short circuit between the positive and negative electrode plates therein occurs, sometimes causing the battery to catch fire or explode.
The conventional lithium polymer battery also has a poor heat radiation characteristic, thereby shortening the effective lifetime of the battery. That is, the pouch type casing cannot actively cope with heat generation occurring during charge/discharge of the battery because its surface is basically formed with nylon or polyethyleneterephthalate (PET) which lowers heat radiation performance. Also, the discharge amount is larger with the increase of temperature, so that the effective lifetime of the battery is rapidly reduced.
Furthermore, if temperature of the battery rises above critical temperature due to heat generation from the battery as stated above, the electrode assembly or the electrolyte may decompose generating a large quantity of gas causing the flexible casing to swell. In addition to the internal heat generation, the swelling of the casing may be increased by external heat supply.
In conventional lithium polymer batteries, the casing uses a metal plate as a core layer. However, since this metal plate is exposed outwardly along circumferences of the casing, it may cause a short circuit between a protective circuit board or a conductor of the external set and the metal plate.
Accordingly, there is a need for a lithium polymer battery which is not easily deformed or pierced by an external force and a battery having a swelling-resistant casing. There is also a need for a lithium polymer battery which has good heat radiation performance and does not cause a short circuit with respect to a protective circuit board, an external set, etc.